As earlier technology of this kind, there is a system of estimating a tire friction state from deviations between lateral acceleration and yaw rate calculated from a vehicle model and actual lateral acceleration and actual yaw rate (cf. patent document 1). In accordance with the estimated tire friction state, this system estimates a vehicle state or controls a vehicle behavior.
Another system is arranged to plot a point corresponding to actual wheel slip rate and a road surface friction coefficient in a two-dimensional map having a horizontal axis representing the wheel slip rate and a vertical axis representing the road surface friction coefficient, and to estimate the tire friction state from the slope of a straight line passing through the plotted point and the origin (cf. patent document 2). In accordance with the estimated tire friction state, this system controls the wheel longitudinal (drive/brake) force.    Patent Document 1: Published Japanese Patent Application Pub. No. H10-44954    Patent Document 2: Published Japanese Patent Application Pub. No 2006-34012